In my experience, a micromanager micromanages for three main reasons.
First, they were once the subject matter expert on the particular task they now are responsible for managing. The micomanager's manager must mentor the necessary skills for strategy-management, so the alleged 'micro'manger can mentor the people performing the tasks. If the 'micro'manager is mentoring on the skill set to adequately perform the task, the 'micro'manager will not be so inclined to jump in. So, the micomanager's manager also shares the responsibility for this behavior. If I am mentored, I am more likely to mentor.
Second, the manager has lost confidence in the person performing that task. The lost confidence could be for a number of reasons, inexperience, dis-engagement, small team, etc., but nonetheless, there is low or no confidence the task will be performed correctly in order to fit properly into the strategy the 'micromanager' is ultimately responsible for. Therefore manage over mentor creeps in opposed to mentor over manage.
Third, the micromanager is not a manager, they are a doer. They have little to no experience managing people and no one is mentoring them to be a manager. But because either they 'stuck around' long enough to get the management job, or they were 'really good' at their task-job; they found themselves sitting in the management seat. Either way, they are the wrong person in the wrong seat in a critical stage in their career and more than likely a critical stage of the business.
In my company (Mojo Media Labs, Marketing Candy), our managers have a tendency to mico-mentor. I have never had anyone complain about being micro-mentored; unless they do not value personal development, but personal development is another story.